A subscriber line interface circuit (SLIC) is an analog integrated telephone line circuit which, in modern telephone systems, replaces a line transformer used in earlier systems. SLICs are mounted on so called line interface boards (LIB) in telephone switches and act as the interface between the telephone line/subscriber and the telephone switch.
The object of a SLIC is i.a. to supply the telephone line with current, detect whether the telephone is off-hook or on-hook, and to disconnect the ringing signal from the line when the called subscriber goes off-hook.
Normally, there are between two and sixteen SLICs on each line interface board. The great advantage of SLICs is that they are physically small and thereby do not require as much space as a transformer. This is particularly valuable since the number of subscribers continuously increases.
The SLICs have a digital control interface which makes it possible to control in which operating mode the respective SLIC is to operate. The operating mode of a SLIC is traditionally controlled from a control circuit, e.g. a microprocessor, via a direct line and three parallel data input lines. This means that, normally, each SLIC requires four dedicated lines from the control circuit. Since one SLIC is required for each subscriber, the lines on the line interface board where the SLIC are mounted, will occupy quite a space on the line interface board.